Land-based seismic surveying uses an array of seismic sensors deployed on the earth's surface in an area of interest. One or more seismic sources (e.g., vibrators, dynamite shots, etc.) generate seismic source signals that travel through the earth, reflect at discontinuities and other features of subsurface formations, and travel back toward the earth's surface. The seismic sensors coupled to the earth at the surface then detect the reflected source signals, and a recording unit records the detected signals. Processing of the recorded signals can then be used to image the subsurface for analysis.
Land-based seismic surveys usually do not record extraneous information to assist in the characterization of a shallow earth model. At most, uphole information is usually recorded at shallow shot/dynamite holes, and that information is then used to improve the shallow “statics” model. Overall, this approach is less than ideal and can be improved.
Microseismic monitoring uses an array of seismic sensors deployed in a wellbore or on the Earth's surface to detect seismic energy emanating from various seismic events occurring within the subsurface. Processed signals from the sensors can identify the position of the event in the subsurface and the time the event took place. In turn, this information can be used in a number of applications to determine movement along faults in rock layers or formations, movement of fluid in a reservoir, monitoring hydraulic fracturing operations, etc. In the end, analysis of the information can be used in well completion and production operations.
A typical form of microseismic monitoring uses an array of sensors (i.e., geophones) deployed downhole in an observation well, which is preferably located close to a well being monitored. For example, FIG. 1 shows a system for determining the distribution and orientation of natural fractures in a target well 12. A source 11 pumps fluid for a hydraulic fracturing operation or the like in the target well 12, which extends below the earth's surface 13 into a fluid or hydrocarbon reservoir 14. The applied pressure from the pumped fluid causes movement along natural fractures in the well 12, producing a microseismic event 17. Seismic waves 18 radiate outwardly from the fracture toward an observation well 21 located within several thousand feet of the target well 12.
Multiple sensors (i.e., geophones) 22 deployed in a vertical array in the observation well 21 detect the waves 18 from the event 17, and a data recording device 24 records the detected signals. Using various algorithms, a signal processor 25 then processes the recorded signals and determines the arrival times of compressional (P) and shear (S) phases of the seismic event 17 to the sensors 22 so the event's hypocenter can be located in the target well 12. See e.g., U.S. Pat. No. 5,996,726. As expected, drilling an observation well can be costly, and the availability of one or more existing wells for use as observation wells within a suitable distance—usually within 1000 m—may be unlikely in most cases.
Another approach to microseismic monitoring uses an array 10 of surface-based sensors (i.e., geophones) 12 as shown in FIG. 2. The array 10 can be arranged to monitor a hydraulic fracturing operation in a vertical wellbore 15 using a pattern of the seismic sensors 12 above the area of interest surrounding the wellbore 15. In response to microseismic events, the sensors 12 detect signals related to seismic amplitude, and a recording unit 14 records the signals for processing.
The array 10 has a hub and spoke form. The sensors 12 in the arms of the array 10 can be spaced at tens of meters from one another, and the arms can extend several thousand meters in length. Because the array 10 is arranged at the surface, there is no need for an observation well. In addition, the array 10 can be distributed over a large area of interest.
Because a microseismic event is detected at the surface, surface noise can be rather large compared to the small event downhole. To overcome the signal weakness compared to noise, the surface array 10 is beam steered so points of greatest energy in the subsurface can be identified. To do this, travel time corrections for subsurface target points are calculated, and the trace data of the surface sensors 12 is time shifted. The data for each target point is stacked so a search of the energy distributions in the subsurface can then give the locations of likely microseismic events. In essence then, this technique attempts to detect events by stacking the seismic data at an arbitrary starting time t0 for the event using a velocity model and stacking. See e.g., U.S. Pat. Publication No. 2011/0286306 to Eisner et al. It should be noted that the stacking procedure using beam steering can fail to detect events because the polarity of a microseismic event may not be uniform across the seismic array 10.
Detecting and locating the microseismic event becomes less reliable as noise increases, and differentiating real events (i.e., fractures, earth shifts, etc.) from false positives becomes more difficult. In fact, the array 10 of surface sensors 12 can fail to detect microseismic events caused by perforations or fracturing operations when there is significant surface noise. Although the array 10 of sensors 12 can facilitate imaging the seismic data, the ultimate uncertainty of whether a real microseismic event has been detected makes it difficult to know that what is imaged is an actual event and not just a false positive.
An approach to passive seismic surveying is illustrated in FIGS. 3A-3B. In this approach, wellbores 10 are drilled to a selected depth of about 100 meters or less and can be drilled deeper when there is very high levels of surface noise. Vertically-arranged arrays of seismic sensors (i.e., single component or three component geophones) 12 suspended on a cable 16 are placed into each wellbore 10, which is then filled. FIG. 3B shows how the wellbores 10 are arranged in two-dimensions over the surface.
When a naturally occurring or induced microseismic event 13 occurs in the subsurface volume, the sensors 12 detect the seismic energy for recording by a recording unit 14. The signals detected by each sensor 12 are recorded for a selected period of time, and a processor processes the signals to beam steer the response of the sensors 12 to enhance signal detection and to reduce noise. For example, each array of sensors 12 in a wellbore 10 is beam steered along a predetermined direction, and the beam steered signals from each vertical array of sensors 12 are combined.
The beam steering is repeated to focus the response of the array to each point in the subsurface to be evaluated for microseismic events. From this, position and time of origin for the microseismic events can be identified.
The beam steering is performed by adding a time delay to the signal recording from each sensor 12. In this way, any event that may have occurred at a specific time at a specific location would be expected to reach the sensor 12 at that associated delay time. Therefore, the time delay applied to the signals depends on the geodetic position and depth of each sensor 12. Additionally, the time delay also depends on the spatial distribution of seismic velocity of the formations in the subsurface, which is determined beforehand by active source reflection seismic surveying and combined in some cases with acoustic measurements made from wellbores penetrating the rock formations to the target depth. See e.g., U.S. Pat. No. 7,663,970 to Duncan et al. and U.S. Pat. Publication No. 2011/0242934 to Thornton et al.
Although the above microseismic approaches may be effective, it will be appreciated that significant variability exists in a subsurface formation at all scales, and the variability directly affects what and how production can be achieved. For example, experience shows that production along a lateral section of a well is not uniform. In fact, any resulting production from a reservoir tends to come from those stages that have been fractured, which may not even include all of the hydraulic fracture stages. Being able to more fully understand and characterize the high spatial variability of a reservoir will always be an ultimate goal in the well completions industry. To that end, microseismic monitoring has the ongoing challenge of detecting and recording small signals in a high-noise environment, accurately locating microseismic events, and mapping those events over a wide area.
The related art discussed above with reference to FIG. 1 through FIG. 3B is not necessarily prior art for the purposes of patentability. The related art is merely discussed as background with respect to the subject matter of the present disclosure.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.